A friction material to be used for brakes such as disc brakes and drum brakes, clutches or the like, which are used for automobiles and the like, is generally formed of materials such as a friction modifier for imparting a friction function and adjusting its friction performance, a fiber base material for exerting a reinforcing function and a binder for integrating these components to impart strength. The friction material has a role of converting kinetic energy to thermal energy by frictional engagement with its opposite material, so that excellent heat resistance, wear resistance, high friction coefficient and stability of the friction coefficient are required. Further, it is also required that the squeal is less likely to occur (squeal characteristics) and the like.
As the friction modifiers for modifying the friction characteristics of the friction material include inorganic or organic fillers, abrasives, solid lubricant and the like are exemplified. These have the respective characteristics, and it is difficult to satisfy all requirements by one kind thereof. Accordingly, two or more kinds thereof are usually used in combination.
Of the friction modifiers, alumina, silica, magnesia, zirconia, copper, aluminum, zinc or the like are utilized as the inorganic fillers (inorganic friction modifiers).
Further, in order to stabilize the friction coefficient of the friction material or to improve wear resistance thereof, a friction material in which a metal is blended has been studied. Patent Document 1 discloses a friction material obtained by forming and curing a friction material composition containing a fiber base material, friction modifier and a binder, in which a bronze powder is contained as the friction modifier. Patent Document 2 discloses a friction material containing Fe2O3 and at least either one of non-whisker-like alkali metal titanates and non-whisker-like alkaline-earth metal titanates.
However, both of the friction materials described in Patent Documents 1 and 2 contain a copper component such as the bronze powder or copper fiber, and when such a friction material is used, copper is contained in a abrasion powder generated during braking, so that there is a possibility of polluting the environment.
Then, as a friction material that is low in the content of the copper component, Patent Document 3 discloses a friction material in which the content of copper is suppressed to a certain value or less and lithium potassium titanate and graphite are contained as friction modifiers.
Further, in order to improve the mechanical strength or heat resistance of the friction material, it is known that a metal fiber or metal particles are blended in the friction material. Patent Document 4 discloses a non-asbestos friction material obtained by forming and curing a friction material composition mainly containing a fiber base material containing steel fiber, a binder and a filler, in which petroleum coke having an average particle size of 50 to 150 μm and hard inorganic particles having an average particle size of 5 to 30 μm are contained. Patent Document 5 discloses a friction material containing a high-melting-point metal powder, a low-melting-point metal powder having a melting point of 420° C. or less, which is lower than that of the high-melting-point metal powder, and a metal sulfide powder.
Furthermore, in recent years, it has been suggested that the friction material containing a copper component such as copper or a copper alloy has a possibility of polluting the environment, because copper is contained in the abrasion powder generated during barking. Accordingly, a friction material containing no copper component has been demanded, in order to exert no adverse effect on the environment while maintaining high performance as the friction material.
As a friction material that is low in the content of the copper component, Patent Document 6 discloses a friction material in which the content of copper and the content of metals other than copper and copper alloy are suppressed to a certain value or less, and a specific amount of an acrylic elastomer-dispersed phenol resin as a binder and a specific amount of Ca(OH)2 and/or CaO as an inorganic filler are contained as essential components.